Polymeric films are used in a wide variety of packaging applications, including food packaging, pharmaceutical products and non-perishable consumer goods. Films suitable for each of these applications are typically required to exhibit a range of physical properties. Food packaging films in particular may be required to meet numerous demanding performance criteria, depending on the specific application. Exemplary performance criteria include outstanding dimensional stability, i.e. a high modulus at both room and elevated temperatures, superior impact resistance, especially at low temperatures, and good transparency.
Horizontal and vertical form-fill-seal processes (HFFS and VFFS, respectively) are particularly rigorous food packaging applications. HFFS is commonly used to form flexible packaging for hot dogs, lunch meats and the like. In HFFS packaging, foodstuffs are introduced into multiple container-like pockets that have been formed across the width of a continuous roll of film (“the forming film”). The pockets are initially thermoformed and then filled as the forming film is continuously transported down a single production line. A second film (“the non-forming film”) is unwound and superposed over the forming film after it has been filled. The two films are then heat sealed at the flat surfaces surrounding the perimeter of each of the forming film pockets. The sealed pockets are then severed at the bonded flat surface, thus forming a final product suitable for sale.
The non-forming film protects the foodstuff in the forming film pockets and is also typically used to identify the product. The non-forming film is thus generally printed with labeling and the like. If the non-forming film lacks adequate dimensional stability the package may become distorted during either the heat sealing, unwinding, or severing processes. Heat sealing processes are especially demanding because they commonly involve drawing a vacuum around the foodstuffs and/or sealing foodstuffs protruding above the forming film pocket. Elevated temperatures are typically used to ensure an adequate bond between the forming and non-forming films, further exacerbating the potential for film deformation. Deformed forming films may stretch and conform to the shape of the product rather than bridging the contours of the foodstuff, distorting the product labeling.
Outstanding dimensional stability is also required for VFFS packaging films used with “hot fill” products, such as soups, sauces, jellies, beverages and other liquefied foods. In hot fill VFFS, flowable foodstuffs at elevated temperatures are introduced through a central, vertical fill tube and into a formed tubular film that has been heat-sealed transversely at its lower end. After being filled, the package, in the form of a pouch, is completed by transversely heat-sealing the upper end of the tubular segment, and severing the pouch from the tubular film above it, usually by applying sufficient heat to melt through the tube above the newly formed upper heat-seal. If the film from which the VFFS package is made does not have sufficient dimensional stability, the package may become distorted either from the heated product or the heat-sealing.
Dimensional stability, particularly high temperature dimensional stability, is also desirable in lidding stock for semi-rigid and rigid containers. Lidding films are commonly used in conjunction with semi-rigid vacuum and/or gas-flushed packages for meat and poultry contained in a foam or other semi-rigid type tray. Lidding films may also be used in rigid packaging constructions, such as packaging for yogurt, custard and other dairy products contained in a rigid cup-like container. When lidding films are applied to such semi-rigid and rigid packages, heat is generally used to seal the film to the container, tray, or cup in which the product is contained. Without sufficient high temperature dimensional stability, the lidding films can stretch during the lidding process, resulting in distorted printed images on the films.
As mentioned above, non-forming films are commonly printed with labeling information. Films exhibiting superior dimensional stability at elevated temperatures are also beneficial in printing processes. Maintenance of color-to-color registration on the printing press is important, as is overall consistency of the “repeat length” of each printed image. Drying tunnel temperatures commonly reach temperatures of 200° F. (93° C.). Films having sufficient resistance to stretching, necking and other types of deformation at elevated temperatures are desirable, so that registration is not lost and the repeat length of the images are consistently maintained on downstream packaging equipment.
Cycloolefinic polymers are known to exhibit outstanding dimensional stability, particularly at elevated temperatures. Cycloolefinic polymers also provide superior optical properties. Cycloolefinic polymers are generally described in numerous United States patents, including U.S. Pat. Nos. 4,948,856; 5,331,057 and 5,468,819. Unfortunately, cycloolefinic polymers are brittle, resulting in poor impact resistance. The impact strength of cycloolefinic polymers can be improved by blending the cycloolefinic polymer with elastomeric copolymers, particularly elastomeric copolymers derived from aromatic vinyl. Cycloolefinic resins incorporating elastomeric resins derived from aromatic vinyl are described in U.S. Pat. Nos. 6,090,888; 4,918,133; 4,992,511; 5,218,049 and EP 726,291. Unfortunately, elastomeric copolymers, including those derived from aromatic vinyl, typically reduce the optical properties of the resulting resin to unacceptable levels.
Attempts have been made to improve the optical properties of impact resistant cycloolefinic compositions. The use of matched refractive indices in resin components is generally known to reduce haze. Consequently, resin suppliers generally recommend the elastomeric copolymers having refractive indices matching that of the cycloolefinic polymer. U.S. Pat. No. 6,090,888 discloses the use of elastomers having refractive indices within 0.05 of the cycloolefinic polymer, for example. By matching refractive indices, cycloolefinic resins exhibiting a haze of 10% or lower have been produced, but such optically acceptable resins are further characterized by inferior impact resistance. Consequently, although intuitively appealing, the use of matched refractive indices has yet to provide cycloolefinic resin compositions providing both acceptable optical properties and impact resistance.